1. Field of the Invention
The present invention generally relates to ink jet printhead apparatus and, more particularly, to a three element switched digital drive system for piezoelectrically driving an ink jet printhead.
2. Description of Related Art
A piezoelectrically actuated ink jet printhead is a relatively small device used to selectively eject tiny ink droplets onto a paper sheet operatively fed through a printer, in which the printhead is incorporated, to thereby form from the ejected ink droplets selected text and/or graphics on the sheet. In one representative configuration thereof, an ink jet printhead has a horizontally spaced parallel array of internal ink-receiving channels. These internal channels are covered at their front ends by a plate member through which a spaced series of small ink discharge orifices are formed. Each channel opens outwardly through a different one of the spaced orifices.
A spaced series of internal piezoelectric wall portions of the printhead body separate and laterally bound the channels along their lengths. To eject an ink droplet through a selected one of the discharge orifices, the two printhead sidewall portions that laterally bound the channel associated with the selected orifice are first piezoelectrically outwardly deflected away from the channel to impart an expansive into the channel. The sidewall portions are held in the outwardly deflected position while the rearwardly propagating portion of the expansive pressure pulse reflects off a back wall of the channel and begins to propagate forwardly in the channel. When the forwardly propagating reflected wave returns to its point of origination, the sidewall portions are inwardly deflected past the rest position and into the channel and again held, thereby imparting a second, reinforcing, pressure pulse into the channel. The sidewall portions are then returned to their normal, undeflected positions. This driven inward deflection of the opposite channel wall portions increases the pressure of the ink within the channel sufficiently to initiate the ejection of a small quantity of ink, in droplet form, outwardly through the discharge orifice.
According to a recently proposed drive method for this type of ink jet printhead, top sides of the internal channel dividing wall portions are commonly connected to ground, and the bottom sides of the wall portions are individually connected to a series of electrical actuating leads. Each of these leads, in turn, is connected to a drive system operable to selectively impart to the lead an electrical waveform that sequentially changes (1) from ground to a first driving polarity, (2) from the first polarity to the opposite polarity, and (3) from the opposite polarity back to ground.
When this electrical waveform is imparted to a piezoelectric channel wall portion bounding one side of a selected, and a second electrical waveform of opposite polarity sequence is simultaneously imparted (via another one of the actuating leads) to the opposite piezoelectric channel wall portion, the opposite channel wall portions, by piezoelectrical action, are sequentially deflected (1) outwardly away from the channel that they laterally bound, (2) into the channel to initiate the ejection of an ink droplet therefrom, and (3) back to their starting or "neutral" positions.
To provide the above drive method in an ink jet printhead, the use of an analog type drive system in which analog circuitry, for example, operational amplifiers (or "op-amps"), deliver the desired driving voltages to the sidewall portions of the ink jet printhead has been suggested. To do so, the analog type drive system would generate an analog voltage waveform which moves linearly from 0 volts to a positive peak +V volts. After holding the voltage at the positive peak for a first period of time, the analog drive system would then move linearly to a negative peak of -V volts and, after again holding the voltage at the negative peak for a second period of time, move linearly back to 0 volts. Due to their linear nature, such analog type drive systems tend to produce unacceptably high levels of power dissipation and have, therefore, proven inefficient in use. Furthermore, when utilized as the drive system for an ink jet printhead, such analog type drive systems tend to lower the operating speed of the printhead. Finally, such analog type drive systems require excessive space on the printhead, thereby adversely affecting driver density for the printhead.
It can be readily seen from the foregoing that it would be desirable to provide an improved ink jet printhead drive system that eliminates, or at least substantially reduces, the above-mentioned limitations and disadvantages associated with the drive systems described above. It is accordingly an object of the present invention to provide such an improved ink jet printhead drive system.